(a)
$P\left( A \cup B^{'} \right) = P(A) + P\left( B^{'} \right) - P(A)P(B^{'})$
$\therefore 0.8 = 0.3 + P\left( B^{'} \right) - 0.3P(B')$
$\Rightarrow 0.5 = P\left( B^{'} \right)(0.7)$
$\Rightarrow P\left( B^{'} \right) = \frac{5}{7}$
$\therefore P(B) = 1 - \frac{5}{7} = \frac{2}{7}$